1. Field of the Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to lateral doping profile engineering for improving the trade-off between pixel well capacity and depletion voltage.
2. The Prior Art
A current trend in pixel sensor arrays is scaling the array to increase device density and decrease die size. As pixels scale, well capacity decreases because of the smaller pixel area. A decreased well capacity erodes the dynamic range of the imager. It is known that well capacity can be improved by increasing the pixel area, but this goes against the scaling trend. Well capacity can also be increased by increasing doping, but this has the disadvantage of resulting in increased depletion voltage. Depletion voltage is set by the product specifications, such as operating voltage and process tolerances. It cannot be increased without affecting other parameters. In particular, increasing depletion voltage above specifications contradicts with the requirement of full well depletion under reset operation, required for suppression of kTC noise and for increasing sensitivity. Improving the trade-off between well capacity and depletion voltage is therefore a fundamental challenge in scaled pixels.
A method for forming a photodiode cathode in an integrated circuit imager includes defining and implanting a photodiode cathode region with a photodiode cathode implant dose of a dopant species and defining and implanting an edge region of the photodiode cathode regions with a photodiode cathode edge implant dose of a dopant species to form a region of higher implant dose than the photodiode cathode implant dose.